1. Field of the Invention
This invention relates to batteries, in particular to large capacity secondary batteries comprising multiple cells arranged in a series-parallel structure. More particular this invention relates to multiple cell batteries having current limiting devices to protect the batteries during a cell shorting event.
2. Description of Related Art
In order to achieve desired voltage and capacity, a battery often consists of many cells. A conventional series-parallel scheme is characterized by connecting a string of cells in series together, then connecting multiple of strings in parallel. A conventional parallel-series scheme is characterized by connecting a bank of cells in parallel, then connecting a plurality of banks in series.
As known in elementary physics, cells (or strings of cells) connected in parallel tend to achieve and maintain identical voltages. The spontaneous voltage equalization within a bank simplifies voltage monitor and control processes during normal usage. But it is dangerous in case one of the cells (or strings of cells) develops an internal short. For brevity the cell (or string of cells) containing the short will be called a shorted cell (or shorted string of cells). Other cells (or strings of cells) in the bank or row will be called parallel neighbors. The shorted cell (or string of cells) receives energy from its parallel neighbors, through a spontaneous inrush current, and become more likely to overheat.
In a parallel-series battery structure, multiple banks are connected in series. Inrush current is primarily circling around the bank harboring the short. In a series-parallel scheme, there is only one bank of strings. Inrush current loops around the entire battery. Loop resistance is greater in a series-parallel battery structure than in a parallel-series structure. Thus the inrush current is smaller in a series-parallel structure than in a parallel-series structure. As a result, a shorted cell is less likely to overheat in a series-parallel battery structure than in a parallel-series battery structure. In other words, a series-parallel structure is more robust against internal short, thus safer than a parallel-series structure.
However in normal usage, each string of the series-parallel battery structure needs to be monitored and balanced independently, whereas the parallel-series battery structure can be monitored and balanced as a single string. Thus a series-parallel structure requires more battery management units (BMU) than a parallel-series structure. This economical consideration has lead to popularity of parallel-series scheme over series-parallel scheme, at the expense of safety.